1. Field of the Invention
The present invention relates to a switching-over control method and apparatus for controlling injection pressure, holding pressure or the like in an injection molding machine.
2. Description of the Prior Art
In a conventional technique, a control for switching over filling pressure and holding pressure of a molten resin within an injection molding machine is conducted by setting positions of a screw relative to a cylinder in advance and comparing the set positional values with values detected by screw position sensors. On the other hand, a starting position where the screw is operative for resin supply is determined by the screw stop position upon completion of plasticizing action or measurement operation. For that purpose the set value of the measurement position setter and the positional signal of the position detector for the screw are compared with each other by a comparator, so that when both the values are identical with each other, an ending signal is applied to a pressure control valve, a flow regulating valve, and/or directional regulating valves to thereby stop the movement of the screw. According to a conventional method, a signal representative of completion of the plasticizing action and measurement operation is generated at a position Sm in FIG. 1.
However, the above-described conventional control manner suffers from the following disadvantages. Namely, although the screw should be stopped exactly at the time of preparation of the signals representative of the completion of the plasticizing action and measurement operation, the screws will be rotated through a small angle due to the inertia moment thereof. It is very difficult to keep constant such a rotation due to instability of mechanical loss or other factors. Furthermore, the pressure distribution of the molten resin in the cylinder is shown in FIG. 1. Assuming the pressure at the screw tip end is expressed by P1 and the pressure on the screw flight side is expressed by P2, the relationship, P1&gt;P2, is established during screw rotation and after the completion of the measurement, so that the molten resin will flow to the left and will gradually move to meet the requirement, P1=P1, and stop. During a period from P1&gt;P2 to P1=P2, the screw will be moved by a distance (Sm'=Sm), by the reaction of the molten resin as shown in FIG. 1 to increase the screw stroke. Namely, as shown in FIG. 1, the total increment of resin amounts corresponding to the inertia rotation of the screw and the resin reaction is expressed by d. As a result, as shown in FIG. 1, the deviation d is added to the stroke from a predetermined metering position Sm to the filling pressure and/or speed change-over position Si, and the strokes to the holding pressure change-over position Sh (Sm-Si, Sm-Sh), respectively. Namely, since the amount of resin to be filled in the mold cavity is changed in accordance with the deviation d, there is a problem in obtaining a precise product.